1. Field
This invention relates to mixing and aeration impellers. Specifically this invention provides a method and apparatus for mixing and aerating mediums, includng slurries processed in flotation machines to recover mineral values therefrom.
2. State of the Art:
In recent years the mineral processing industry has found it necessary to treat ever increasing tonnages of lower and lower grade ores. This, in turn, has dictated the use of correspondingly increasingly large processing equipment, including flotation machines.
A slurry is prepared from a ground mixture of ore and a liqid which is typically water with selected conditioning, collecting and frothing agents. The slurry is fed into a flotation machine comprised of one or more flotation cells. In the flotation cells, means are typically employed to both aerate and physically mix the slurry. Aeration and mixing are desired to simultaneously produce bubbles in the slurry and to bring ore particles in contact with the bubbles. As is known, ore particles having desired mineral values tend to be carried to the surface of the slurry to form a forth which may be regarded as a concentrate of the desired mineral value to be recovered. As known, in the flotation process, some particles, which tend to be the heavier particles which are frequently rich in the desired mineral value, tend to settle to the bottom of the flotation cell. In order to improve the efficiency of the flotation machine process it is desirable to thoroughly mix and suspend all particles in the slurry without overmixing or agitating which could reduce the opportunity for bubbles to transport desired particles to the surface (froth).
Heretofore the mixing operation within the flotation cell has been effected by a variety of mixing means. For example, a ship-type propeller could be used within the slurry. It would be rotated to produce a generally downward flow and in turn generate a circulating current which is deflected from the bottom of the cell. However, such a propeller would not usually be reversible to vary the flow currents within the cell to minimize the collection of solids near the bottom of the cell. Such limitation would be more severe when the ship-type propeller is used with means for the introduction of gas to enhance mixing. Further, the flow currents above the propeller are less pronounced. In turn, mixing is less effacious. Moreover, solids could build-up on the cell bottom in the flow vortex. A ship-type propeller may be more readily subject to the build-up of corrosion and precipitates on the propeller blades.
Turbine-type impellers have been used in flotation cells and can be either unidirectional or reversible (in rotation) depending upon the design of the impeller blades. However, the flow patterns through such an impeller are such that the bottom of the cell cannot be readily swept by flow to prevent the build-up of solids on the bottom which settle out from the slurry.
Modern aeration impellers in common use (e.g., those manufactured by the Galigher Co. of Salt Lake City, Utah, and sold under the trademark "AGITAIR") can be reversed to improve impeller life. Flow through such impellers is generally upward under and downward to the impeller and then radially outward. The flow currents along the bottom of the cell are not pronounced. In turn, mixing is less thorough because a direct outward flow of fluid is not directed at the bottom to sweep it clean and keep the heavier particulate matter suspended in the slurry.
Injecting gas into such an impeller is effected so that a slurry gas mixture is pumped by the turbine blades. The gas fluid interfaces tend to accelerate erosion and, when the gas is chemically capable of contributing to corrosion (e.g., air), corrosion of the impeller. Further, the gas tends to decrease the efficiency of the impeller because it inhibits laminar flow and by its mere presence reduces the volume of slurry processed.
The Outokumpu OK-16 pneumatic flotation machine uses an impeller such as the one shown on page 148 of Mining Magazine, August 1976 (published by Mining Journal Ltd, London, England). Gas is directed into the fluid before it passes through the impeller blades. Impellers of this type also suffer increased erosion and corrosion across the turbine blades by virtue of the presence of the gas which is generally compressed air. Further, such an impeller does not direct a flow of slurry to the bottom to sweep the bottom of the cell. Therefore, not all matter is suspended within the slurry for processing. The processing of the slurry throughout the system is thus less efficient and in turn less economical.
U.S. Pat. No. 3,843,101 (Green) also discloses an impeller which may be regarded as a flotation machine impeller.